Always go new tech if buying new.
I suggest I5-6600K and plan to overclock.
It will perform about the same in gaming as a I7-6700K and be somewhat cheaper.
You will need a Z170 motherboard to overclock. Any will do.
Skylake needs low voltage ddr3, 1.35v. Unlikely your ddr3 will work.
You will want ddr4, 2133 speed is fine.
Sometimes, your old windows C drive will boot to a new motherboard.
If so, you task is easy, you only need to install the new drivers that come with your motherboard.
There are some things you can do to prepare for such a change; google is your friend there
Here is one, for example:
http://www.makeuseof.com/tag/making-sure-windows-7-will-boot-after-changing-the-motherboard/
If you need to do a clean install, your old apps that used the registry will need to be reinstalled.
Then, there may be an issue if your windows copy was OEM.
That makes it tied to your original motherboard.
MS is lenient on letting you reactivate though it might require phone activation.
If all else fails, you can buy a new license.